2. Field of the Invention
The present invention relates to a cathode-ray tube (hereinafter referred to as CRT) in which an anti-reflection film, anti-static film, and film for screening the CRT from a leakage electric field (VLF band width) are provided on the surface of its face plate.
2. Description of Related Art
Because of its principle of operation, a high voltage over 20 kV is applied to the phosphor screen of a CRT in order to accelerate an electron beam. As higher luminance and resolution have been realized in recent years, a high voltage of 30 kV or more is applied in a CRT for a color television. Even in a CRT for a display monitor, a voltage as high as 25 kV is applied. When the power source for the associated set is turned on, the outer surface of the face plate of a CRT charges up, so that a discharging phenomenon may occur when the viewer comes close to the CRT, thus causing an uncomfortable sensation or an electrical shock to the viewer.
In order to prevent such a phenomenon, a coating film having a surface resistance value of about 10.sup.9 .OMEGA./.quadrature. is conventionally formed on the face plate, or a glass panel provided with a conductive film having a surface resistance value of about 10.sup.9 is bonded to the surface of the face plate by means of a UV (ultraviolet) curing resin having substantially the same refractive index as that of the glass panel, so that a part of the coating film or conductive film is grounded via a metal anti-explosion band wound around the face plate, thereby causing a discharge.
FIG. 1 is a side view schematically showing a conventional CRT of anti-static-processed type, which is provided with the function of preventing a static-electrical charge mentioned above. In the drawing, numeral 1 denotes a CRT, and on the face plate section 3 formed on the front face of the CRT 1 is provided a glass panel 2 having a conductive film via a UV curing resin. The glass panel 2 may be composed of a rough conductive film 2 formed on the surface of the face plate section 3.
The side portion of the CRT 1 constitutes a funnel section 4 which is provided with a high-voltage button 5 in the upper part thereof. The back portion of the CRT 1 constitutes a neck section 6 in which an electron gun (not shown) is built. Over the boundary between the funnel section 4 and neck section 6 is fixed a deflection yoke 7. The high-voltage button 5, electron gun, and deflection yoke 7 are connected to a high-voltage power source 35, driving power source 36, and deflection power source 37 via lead wires 5a, 6a, and 7a, respectively.
Around the side face of the face plate section 3 is provided the metal anti-explosion band 9, which is fixed thereto by means of a conductive tape 8 provided around the glass panel 2. The conductive tape 8 may be substituted with a conductive paste. To the metal anti-explosion band 9 is attached a mounting lug 10, which is connected to the ground 12 via an ground wire 11. The glass panel 2 having the conductive film is connected to the ground 12 via the conductive tape 8, anti-explosion band 9, mounting lug 10, and ground wire 11, so that the charge is constantly connected to the ground 12.
In the CRT 1 thus constituted, an electron beam emitted from the electron gun which is built in the neck section 6 is electromagnetically deflected by the deflection yoke 7, while a high voltage is applied onto the phosphor screen provided on the inner surface of the face plate section 3 via the high-voltage button 5.so as to accelerate the electron beam. The resulting energy of the accelerated electron beam excites the phosphor screen to emit light, thus obtaining a light output.
As described above, the outer surface of the face plate section 3 charges up under the influence of the high voltage applied to the phosphor screen provided one the inner surface of the face plate section 3, so that a discharging phenomenon occurs when the viewer approaches the face plate section 3, thus causing an uncomfortable sensation or electrical shock to the viewer. The charging up also causes fine particles of dust in the air to land on the outer surface of the face plate section 3, resulting in visible contamination that deteriorates the image quality.
To overcome such problems, conductive coating is provided on the outer surface of the face plate section 3 or a glass panel provided with a conductive film is bonded to the outer surface of the face plate section 3 by means of a UV curing resin having substantially the same refractive index as that of glass, as shown in FIG. 1. By connecting the conductive films to the ground 12, the charge is always allowed to escape to the ground, thereby preventing the charging up of the outer surface of the face plate section 3. For such a CRT of anti-static-processed type, it is sufficient to have a surface resistance value of about 10.sup.9 .OMEGA./.quadrature.. Therefore, a material which contains fine particles of antimony-containing tin oxide as a filler has been used for coating.
Moreover, since a CRT generally reflects external light on the surface of its face plate, it presents another problem that images displayed thereon are hard to be seen by the viewer. As a means to overcome the problem, such an anti-glaring treatment is performed. According to the treatment, an uneven surface configuration is imparted to the foregoing conductive film so that the external light incident upon the surface of the face plate is irregularly reflected. Due to the uneven configuration, however, not only the external light incident upon the surface of the face plate but also the light emitted from the phosphor screen are irregularly reflected, resulting in the deterioration of the resolution and contrast of images displayed.
The glass panel 2 provided with the conductive film is typically composed of four optical thin films (of which the lowermost layer is composed of the conductive film). These four optical thin films, which are made of materials having different refractive indices, are formed by vapor deposition in such a manner that films with a high-refractive index and films with a low-refractive films are alternately stacked so as to provide, e.g., a layered structure of high-refractive index/low-refractive index/high-refractive index/low-refractive index, thereby lowering the surface reflectance. In addition, by maintaining the resistance value of the lowermost conductive film at 3.times.10.sup.3 .OMEGA./.quadrature. or less, the CRT can be screened from the leakage electric field (VLF band width). Since the four optical thin films are smooth films formed by vapor deposition, they do not deteriorate images displayed and exert sufficient low-reflective effect. However, their material and production cost is increased and their weight is also increased because of the UV curing resin employed for bonding the glass panel to the face plate section.
On the other hand, there has recently been initiated the practical use of a double-layer low-reflective coat, which is obtained by directly coating the face plate section of a CRT. Since the double-layer low-reflective coat is a smooth film, it is free from the deterioration of the resolution and contrast of images displayed. However, it cannot provide the sufficient low-reflective effect so that the contours of reflected images are disadvantageously sharpened. Furthermore, since visible fingerprints are easily Left on the coat, it should have sufficient film strength and, in particular, abrasive resistance to withstand a cleaning process for removing the fingerprints.
The method of producing the double-layer low-reflective coat is subdivided into a method of forming the first high-refractive conductive layer by chemical vapor deposition (hereinafter referred to as CVD) and forming the second layer by spin coating and a method of forming the first and second layers By spin coating. The former CVD technique requires a heating process to elevate the temperature of the face plate section to about 500.degree. C., so that it is not applicable to a post-process performed with respect to a finished CRT. Next, the method of forming the first and second layers by using a spin-coating technique, which can be applicable to a post-process performed with a finish CRT, will be described below.
FIG. 2 is a flow chart illustrating the production process using the spin-coating technique. As shown in the flow chart, the face plate section of a finished CRT is preheated to 40.degree. to 50.degree. C. in a furnace (step S11), and then carried into a first spin booth. In the spin booth are disposed a spinner, coating-solution dispenser, and the like. The spin booth is provided with a function of adjusting the inside temperature, humidity, and dust level. The face plate section of the finished CRT, which has been carried into the spin booth, is spin-coated with a solution for the first layer containing tin oxide (SnO.sub.2) which is a conductive material of high-refractive index, silica (SiO.sub.2) for forming the film, and an alcohol serving as a solvent, thus forming the first high-refractive conductive layer (step S12).
After performing a drying and curing process at a temperature of about 100.degree. C. (step S13) and then lowering the temperature to 40.degree. to 50.degree. C. (step S14), the CRT is further carried into a second spin booth in which the face plate section is further spin-coated with an alcoholic solution for the second layer containing silica (SiO.sub.2) as a low-refractive transparent material, thus forming the second low-refractive transparent layer (step S15). The high-refractive conductive layer and low-refractive transparent layer are then cured by baking at 150.degree. to 200.degree. C. in the furnace, thus forming a CRT with the double-layer low-reflective coat (step S16). The second spin booth is provided with the same function as that of the first spin booth.
In the conventional method described above, the first and second spin booths are independently provided, and the furnace for the drying, curing, and temperature-lowering process after applying the first layer is required, which increases the equipment cost and process steps.